T cell tolerance to tumor-associated antigens is a significant barrier to immune based treatments of human cancers. One such tumor-associated antigen is the proto-oncogene HER-2/neu (neu) which is overexpressed in 35-40% of all human breast cancers. Although patients with neu expressing tumors develop antibody and T cell responses to this antigen, these responses are weak and unable to hinder tumor growth. Our work has focused on understanding these mechanisms of T cell tolerance using the neu-N transgenic mice that express the wild type rat neu cDNA under control of the MMTV promoter. Since neu is an endogenously expressed antigen, profound neu-specific immune tolerance exists in the neu-N mice. We previously reported the immunodominant T cell epitope of neu recognized by parental FVB/N mice, RNEU420-429. We have investigated whether altering RNEU420-429 can generate a more immunogenic peptide that will result in better protection from a HER-2/neu expressing tumor in the neu-N mice. Also, using GFP-expressing RNEU420-429-specific T cells, we demonstrate that high avidity CTL cannot persist in the periphery of neu-N mice but do persist in the periphery of parental mice. Further studies are underway to understand the role other immune cells (such as CD4(+)CD25(+) regulatory T cells) play in CD8(+) T cell tolerance. This work to further understand the mechanisms of T cell tolerance in this cancer model should lead to even further improvements in vaccination strategy for cancer immunotherapies.